Musahl C, Holthoff H P, Lesch R, Knippers R
Department of Biology, Universität Konstanz, Germany.
Exp Cell Res. 1998 May 25;241(1):260-4. doi: 10.1006/excr.1998.4041.
Mcm proteins are abundant nuclear proteins involved in the regulation of genome replication. Previous experiments had shown that levels of Mcm-specific mRNAs increase at the G1/S phase transition of the cell cycle, but that the amounts of Mcm proteins do not change much during the cell cycle. To learn more about the stability of an Mcm protein we performed experiments which showed that: (i) more than 60% of [35S]methionine pulse-labeled Mcm3 protein appears to be degraded during a 24-h chase in HeLa cells; (ii) the amount of Mcm3 protein significantly decreases during the differentiation of HL60 cells in vitro (whereas another replication-initiation protein, hOrc2, remains fairly constant); and (iii) according to immunohistochemical staining, Mcm3 protein is present in nuclei of cells in the proliferating zone of human epidermal tissue, but in decreasing amounts in nuclei of differentiating cells of the upper cell layers. Our interpretation is that Mcm3 protein is no longer synthesized after initiation of differentiation and slowly disappears at a half-life of approximately 24 h.
Mcm蛋白是丰富的核蛋白，参与基因组复制的调控。先前的实验表明，Mcm特异性mRNA的水平在细胞周期的G1/S期转换时增加，但Mcm蛋白的量在细胞周期中变化不大。为了更多地了解Mcm蛋白的稳定性，我们进行了实验，结果表明：（i）在HeLa细胞中进行24小时追踪期间，超过60%的[35S]甲硫氨酸脉冲标记的Mcm3蛋白似乎被降解；（ii）在HL60细胞体外分化过程中，Mcm3蛋白的量显著减少（而另一种复制起始蛋白hOrc2保持相当恒定）；（iii）根据免疫组织化学染色，Mcm3蛋白存在于人类表皮组织增殖区细胞的细胞核中，但在上层细胞分化细胞核中的量逐渐减少。我们的解释是，Mcm3蛋白在分化开始后不再合成，并以约24小时的半衰期缓慢消失。
